Many electrical and electronic devices utilize rechargeable or "secondary" batteries as power sources. One type of rechargeable battery, comprising nickel-cadmium electrodes or "NiCad" type battery, is used as a source of portable or alternate power in environments requiring a sealed battery and where battery maintenance must be minimized. While NiCad batteries are relatively expensive, a relatively simple constant current supply is used for charging and recharging.
In heavy current demand applications where battery size and cost must be minimized, lead-acid batteries are often used. Improvements in construction, including the capability to make a sealed lead-acid battery, have made possible the use of lead-acid batteries in low maintenance environments and in equipment having relatively low current demands. Technical information on sealed lead-acid batteries is available in publications such as The Sealed Lead Battery Handbook published by the General Electric Corporation. In comparison to NiCad batteries, lead-acid batteries are more economical at an equivalent energy level.
To maintain a lead-acid battery fully charged, a voltage source is connected to the battery to supply a "float" or "trickle" charge. FIG. 1 is a schematic diagram of a simplified battery charger 10 for a lead-acid battery. Voltage reference 12 is connected to an inverting input of operational amplifier 18 through input resistor 14. The noninverting input of operational amplifier 18 is connected to the positive terminal of voltage reference 12 through ground. An output of operational amplifier 18 is connected to lead-acid battery 20, supplying a charging voltage to the battery. Feedback resistor 16 provides a negative feedback signal from the output back to the inverting input terminal of operational amplifier 18 to regulate and stabilize the output voltage of the amplifier.
The voltage gain of operational amplifier 18 as shown in FIG. 1 is equal to the resistance value of feedback resistor 16 divided by the resistance value of input resistance 14. Thus, the charging voltage to battery 20 output by operational amplifier 18 is equal to the input voltage supplied by voltage reference 12 multiplied by the voltage gain of the amplifier, expressed as: EQU V.sub.out =-(R.sub.f /R.sub.i)V.sub.in ( 1)
where voltage reference 12 is V.sub.in, input resistor 14 is R.sub.i and feedback resistor 16 is R.sub.f. For example, if voltage reference 12 is at 5 volts, input resistor 14 has a value of 10K ohms, and feedback resistor 16 has a value of 20K ohms, the voltage gain of operational amplifier 18 is equal to 2 (20K/10K), resulting in an output voltage of 10 volts being supplied to battery 20, as follows: EQU V.sub.out =-(20K/10K)(-5V) (2)
A problem with lead-acid batteries is caused by an increase in chemical reactivity as the temperature of the battery increases; the reactivity doubles approximately every 10 degrees Celsius. As the temperature of the battery increases, the internal resistance decreases so that the battery accepts a larger charging current at a given charging voltage. The increased current flow generates additional heating of the battery, further reducing its internal resistance. This cycle of battery heating followed by an increase in battery charging current results in a run-away condition which can damage the battery and cause it to fail. Therefore, to avoid this problem, it is necessary provide temperature compensation in a lead-acid battery charger to avoid overcharging and thermal run-away of the battery. Accordingly, to achieve a reliable and long life battery/charger system using sealed lead-acid batteries, the float or trickle voltage of the battery charger should be corrected for ambient battery temperature by decreasing the charging or float voltage as temperature increases.
The range of compensation for a lead-acid battery in terms of voltage with respect to temperature is typically from -2.5 millivolts per degree Celsius per cell (mv/deg. C/cell) to -7 mv/deg. C/cell, depending on cell temperature and the desired overcharge rate.
Prior art battery chargers have accomplished voltage temperature compensation using one of two techniques. The battery charger circuit shown in FIG. 2 illustrates one such charger circuit with a negative coefficient thermistor 34 in the negative feedback loop of operational amplifier 18. Resistors 36 and 38 are chosen to provide the desired total feedback resistance in combination with negative temperature coefficient thermistor 34 to produce a desired amplifier voltage gain. As thermistor 34 is subjected to higher ambient temperature, its resistance value decreases, thereby resulting in a lower total feedback loop resistance value. This causes the voltage gain of amplifier 18 to decrease so that the voltage supplied to lead-acid battery 20 is proportionally reduced.
While the circuit of FIG. 2 results in a negative charging voltage-temperature coefficient, thereby protecting lead-acid battery 20, it has the disadvantage of requiring the use of a relatively expensive thermistor in the compensation network. Further, because suitable thermistors are available in limited ranges of resistance values and having limited values of temperature coefficients, it is necessary to change or adjust several charger components to obtain a desired charger voltage-temperature coefficient.
A second prior art temperature compensation technique, is shown schematically in FIG. 3, provides a reference voltage source having a negative temperature coefficient, with voltage source 12, resistor 40 and transistor 42 comprising a temperature compensated voltage reference source. Because the base-to-emitter voltage (V.sub.be) of transistor 42 exhibits a negative voltage-temperature coefficient, the input reference voltage to operational amplifier 18 varies accordingly. However, silicon transistors exhibit a limited range of temperature induced characteristics, having a typical V.sub.be value in the range of 0.6 volt and with a temperature coefficient of approximately -2.2 mV/deg. C. Therefore, amplifier gain must be adjusted to obtain a desired charger voltage-temperature compensation. Further, many commercially available voltage regulator circuits include a self-contained voltage reference source connected directly to an associated operational amplifier in a single integrated circuit package. For example, the National Semiconductor LM1578 series of switching regulators include a fixed internal voltage reference which is inaccessible externally from the integrated circuit package. Thus, it is not possible to provide either a different voltage reference source or temperature compensation circuitry for the on-chip voltage source.
A need therefore exists for a charger having a negative charging voltage-temperature coefficient for lead acid batteries.
A need further exists for a temperature compensated battery charger without using expensive components to provide a negative temperature coefficient of charging voltage.
A need further exists for a temperature compensated battery charger which can readily be adapted to accommodate different battery voltages and voltage-temperature coefficients.
A need further exist for voltage-temperature compensation in battery chargers using conventional voltage reference sources.
A need further exists for a voltage-temperature compensated battery charger using a minimum number of components to provide a negative voltage-temperature coefficient of charging voltage.
Accordingly, an object of the invention is to provide a temperature compensated charger for use with batteries which is readily adaptable for use with various battery charging voltages and voltage-temperature coefficients using a conventional voltage reference source.
Another object of the invention is to provide a temperature compensated charger for use with batteries using inexpensive and readily available components.